Elements taking advantage of a Peltier effect or Seebeck effect are used as thermoelectric conversion elements. Since thermoelectric conversion elements have a simple structure, are easy to handle and able to maintain a stable characteristic, widespread use of thermoelectric conversion elements is attracting attention in recent years. Especially when used as an electronic cooling element, the thermoelectric conversion element can perform local cooling and accurate control over temperature close to a room temperature. And therefore a wide range of studies are being carried for stabilizing temperature of opto-electronics and semiconductor laser or the like.
The aforementioned thermoelectric module for electronic cooling element or thermoelectric power generation is configured as shown in FIG. 11 by connecting P-type thermoelectric conversion element (P-type semiconductor) 5 and N-type thermoelectric conversion element (N-type semiconductor) 6 via a connection electrode (metal electrode) 7 to form a PN element pair, and then arranging a plurality of such PN element pairs in series. One end of P-type thermoelectric conversion element 5 and N-type thermoelectric conversion element 6 is heated and the other end is cooled, which is set based on the direction of a current flowing through each PN element pair. In FIG. 11, reference numerals 8 and 9 denote external connection terminals, 10 denotes a ceramic substrate and H denotes an arrow indicating a heat flow direction.
The material for thermoelectric conversion element has large performance index Z (=a2/rK) expressed by Seebeck coefficient “a” which is a substance-specific constant, specific resistance “r” and thermal conductivity “K”, which are used in the temperature region where the element is used. Crystal materials generally used as thermoelectric conversion elements are Bi2Te3-based materials. These crystals have an outstanding cleavage property, that is, these crystals are known to have a problem that an yield extremely decrease due to cracking or chipping by undergoing slicing and dicing or the like for obtaining a thermoelectric conversion element from an ingot.
To solve this problem, a method of manufacturing a thermoelectric conversion element module is proposed, which includes: heating step of heating/melting a mixing material powders having a desired composition; coagulation step of forming a solid solution ingot of a thermoelectrically conversion material having a rhombohedral structure (hexagonal structure); crushing step of crushing the solid solution ingot so as to obtain solid solution powder; sizing step of uniformalizing the grain size of the solid solution powder; sintering step of sintering the uniformalized solid solution powder under a pressure; and hot upset forging step of hot-pressing and rolling the sintered substance so as to be plastic-deformed, and thereby orienting crystal structure of the sintered substance in a crystal orientation for achieving an excellent performance index or the like (e.g., see Patent Literature 1).
Furthermore, a conventional method of manufacturing a thermoelectric conversion element module includes: manufacturing an alloy ingot; crushing the alloy ingot into raw powder having an average powder grain size of 0.1 micron (um) or above and 1 um or below, under a vacuum with an oxygen concentration of 100 ppm or below or under an atmosphere of inert gas; and sintering the raw powder through electric resistance heating while adding a pressure to the raw powder. In the sintering process, the raw powder is subject to joule heat from a pulsed current and pressure of 100 kg/cm2 or above and 1,000 kg/cm2 or below (9.8 MPa or above and 98.1 MPa or below). A thermoelectric conversion material with a fine crystal grain size and with excellent workability can be manufactured through this method (e.g., see Patent Literature 2).
Furthermore, a conventional thermoelectric conversion element module is known, which includes a thermoelectric conversion element group comprising a plurality of P-type thermoelectric conversion elements and a plurality of N-type thermoelectric conversion elements, a pair of substrates sandwiching the thermoelectric conversion element group, and a connection electrode which is arranged on a surface on the element group side of the substrates and which electrically connects the P-type thermoelectric conversion element and the N-type thermoelectric conversion element in series (e.g., see Patent Literatures 3 to 9). Furthermore, a conventional thermoelectric conversion element module is known, which includes a thermoelectric conversion element group comprising a plurality of P-type thermoelectric conversion elements and a plurality of N-type thermoelectric conversion elements, and a connection electrode which is provided on the end faces of P-type thermoelectric conversion element and N-type thermoelectric conversion element adjacent each other and which electrically connects the elements each other (e.g., see Patent Literature 10).